Anhydrous lithium chloride is used as a raw material in molten salt electrolysis for production of lithium metal, or a desiccant. As a method for producing anhydrous lithium chloride, for example, Patent Publication 1 discloses reaction of lithium hydroxide with hydrochloric acid for producing a high purity product, and Patent Publication 2 discloses reaction of lithium carbonate suspended in water with chlorine gas in the presence of an iron-nickel catalyst.
Both of these methods inevitably include dehydrating and drying of the resulting lithium chloride for obtaining anhydrous lithium chloride, which requires additional costs for operation and facility.
Patent Publication 3 discloses a method for producing anhydrous lithium chloride by reacting chlorine gas with lithium hydroxide. The raw material lithium hydroxide, which is strongly alkaline, irritates eyes, skin, and mucosa, and is easily stirred up when, in particular, the reaction is performed in a dry process, which causes difficulties in handling and additional costs for operation and facility.
When lithium carbonate in the form of molten salt, which is highly corrosive to metal materials, is used in the production of anhydrous lithium chloride, severe limitation is imposed on the materials which may be used for the reaction vessel or piping (Non-patent Publications 1 and 2). In addition, the heat resistance of the reaction vessel should also be considered in view of the high temperature of the molten salt. On the other hand, for the production of anhydrous lithium chloride using chlorine gas, which is also corrosive, there are hardly any metal materials that could be used in the chlorine gas atmosphere beyond 500° C. Instead, ceramics and the like materials which withstand high temperature, corrosive environment, need to be employed for the reaction vessel and piping.
Molten salt electrolysis has been employed for producing lithium metal, and attempts have conventionally been made to use inexpensive lithium carbonate as a lithium source. However, lithium carbonate is not in use in commercial production at present because the graphite anode-consuming electrolytic reaction (2Li2CO2+C→4Li+3CO2) is the main reaction, and the lithium metal resulting from the electrolysis reacts with lithium carbonate (Li2CO3+4Li→3Li2O+C) in the electrolyte to obstruct continuous electrolysis.
In view of the above problems, it is the current practice to produce lithium metal from anhydrous lithium chloride as a lithium source by molten salt electrolysis.
Patent Publication 4 discloses a method of molten salt electrolysis using anhydrous lithium chloride, wherein lithium carbonate is introduced onto the surface of the bath around the anode to cause the reaction 2Li2CO2+2Cl2→4LiCl+2CO2+O2, to thereby generate anhydrous lithium chloride and allow continuous electrolysis.
Patent Publication 5 discloses a method of molten salt electrolysis using anhydrous lithium chloride, wherein lithium carbonate and charcoal or the like as a carbon source are simultaneously introduced into the anode compartment to cause the reaction 2Li2CO2+2Cl2+C→4LiCl+3CO2, to thereby prevent consumption of the anode.
The methods disclosed in Patent Publications 4 and 5 do not solve the problem of reaction between the lithium metal resulting from electrolysis and lithium carbonate discussed above. Thus these methods have difficulties in the control of carbonate concentration and various problems in operation, such as declined current efficiency, black foam, and short circuit.
Patent Publication 6 discloses a method including the steps of extracting a part of a mixed molten salt of the electrolyte containing anhydrous lithium chloride outside the electrolytic cell, introducing the extracted molten salt into a chlorinating furnace, adding lithium carbonate and a chlorinating agent thereto, reacting the molten lithium carbonate and the chlorinating agent, and returning the resulting anhydrous lithium chloride to the electrolytic cell for use as a raw material. This method has difficulties in controlling the concentration of the electrolyte, requires circulation facilities, and is not practical in view of safety.
Patent Publication 7 discloses an electrolysis method wherein the anode compartment is separated from the cathode compartment with a porous electrically nonconductive partition, lithium carbonate is introduced into the anode compartment, and only the lithium ions are delivered to the cathode compartment to deposit lithium metal. This method requires a high temperature, the current efficiency is low, and the corrosion resistance of the nonconductive partition should be attended to.    Patent Publication 1: U.S. Pat. No. 4,980,136-A1    Patent Publication 2: RU-2116251-C1    Patent Publication 3: U.S. Pat. No. 2,968,526-A1    Patent Publication 4: U.S. Pat. No. 3,344,049-A1    Patent Publication 5: JP-59-200731-A    Patent Publication 6: JP-1-152226-A    Patent Publication 7: U.S. Pat. No. 4,988,417-A1    Non-patent Publication 1: “Youyuen Netsugijutsu no Kiso (Basics of Molten Salt Thermal Technology)”, written and edited by The Society of Molten-Salt Thermal Technology, published by Agne Gijutsu Center (1993), p 97    Non-patent Publication 2: “Youyuen no Ouyou (Applications of Molten Salt)”, written and edited by Yasuhiko ITO, published by Industrial Publishing & Consulting, Inc. (2003), p 305